1. Field of the Invention
The present invention relates to a process for the biological treatment of mud and oily residues generated by the oil and petrolchemical industry.
2. Description of the Prior Art
Materials of this kind arise from the storage of crude oil, intermediates and end-produces of an oil origin, from refinery processing and accidental contamination of the ground with hydrocarbons. They are obtained in a great quantity. Recent estimations indicate about 2.5 and 0.8 million tons per year of mud coming from the American and European refining industries respectively. Italian law classifies process mud arising from the storage of oil products as toxic and harmful waste and provides for their placement in third-category type C discharge grounds.
American law (Resource Conservation and Recovery Act) classifies mud generated by refineries as "dangerous" and has adopted a special classification thereof:
tank bottoms for the storage of lead distillates (code EPA K 052); PA1 API separator mud (separating systems for oil-water-solids before water treatment plants, code EPA K 051); PA1 materials obtained from the air flotation of water-oil mixtures (code EPA K 048); PA1 water-oil emulsions (slop oil emulsion, code EPA K 049); PA1 mud obtained from the cleaning of exchangers (code EPA K 050); PA1 primary mud obtained from water storage basins and water-oil separation plants (code EPA F 037); PA1 secondary mud coming from flotation plants and other equipment for the collection of water-oil waste (code EPA F 038). PA1 prevention of pollution by minimizing the volume of mud produced; PA1 recycling of mud to the productive units (cokization plants and catalytic cracking); PA1 thermal treatment, outside the battery limits of the refineries, in centralized incinerators or ovens for cement (off-site treatment); PA1 purification within the battery limits of oil plants (on-site treatment) by extraction techniques with solvent, thermal desorption, biological treatment.
These materials can be disposed of by thermodestruction, landfilling or landfarming. If the latter disposal methods are used, American law (Hazardous and Solid Waste Amendments Act, HSWA 1984) imposes pre-treatment to reach the BDAT standards (Best Demonstrated Available Technology). These standards were promulgated by EPA at the end of the 80s' for K type mud and in 1992 for F type mud. For the latter waste however the adoption of the BDTA limits was postponed until 1994. The limitations involved relate to the main aromatic polycyclic hydrocarbons (anthracene, benzoanthracene, benzopyrene, chrysene, naphthalene, phenanthrene and pyrene), the main heavy metals (chromium and nichel) and organic molecules with a high toxicity (benzene, ethyl and butylphthalate, ethylbenzene, phenol, toluene, xylene, cresols and cyanides).
The impact of the adoption of these new standards on the American refining industry has been considerable. It has in fact been estimated that the costs of treating this waste, indicated at about 40 million dollars in 1992, will rise to 240 million dollars in 1994.
In Europe, the standards which regulate the disposal of waste material contaminated by hydrocarbons are not the same in the various countries. In any case however, for the disposal of waste generated by oil refinery alone, over 38 million dollars are spent each year.
The oil industry is making very severe regulations for environmental protection with the following main approaches:
Among on-site processes, original physico-chemical approaches can be noted. U.S. Pat. No. 4,971,703 (Battelle Memorial Institute), proposes for example, a breaking process of typical oily emulsions such as mud from API separators and air flotation systems. This is based on heating to a high temperature (equal to or higher than 300.degree. C.) and high pressure (higher than water vapor pressure). The treatment is prolonged for such times as to allow the separation of an oily flow which can be recycled in the refinery, a water flow which is sent to the water treatment plants and a small flow with a high content of solids miscible with the waste water flow. Patent EP 372 761 A2 (The Standard Oil Company) and those related to it, propose instead extraction technologies with a solvent of undesired products from refinery mud as such, refinery mud subjected to dewatering and extraction mud of the crude product.
Among on-site treatment techniques, biological ones are generally preferred. They are in fact characterized by minimum environmental discharges with consequent fewer management problems. For example, in the United States the management of biological plants is regulated by federal laws defined by the Clean Water Act. They are consequently exempt from management permits whose issue on the part of the proposed organizations (Resource Conservation and Recovery Act) often influences the economic feasibility of on-site treatment processes. For biological-type processes applications on the following scales are reported: a) laboratory (60 l reactors), b) demonstrative (mobile unit tested at the beginning of 1992 in the Gulf Coast Refinery) and c) industrial (carried out at Amoco's former Sugar Creek refinery, Montana, and the French Limited Superfund, Tex.).
These processes are based on the following scheme. The oily mud (see FIG. 1), under suitable process conditions, is fed to a pre-treatment phase carried out to recover most of the oily phase present and reduce the organic matter being fed to the biological section. The pre-treatment is normally carried out with three-phase decanter centrifuges, or with filtrating systems whose functioning can be facilitated by recycling the treated mud. The bio-oxidation treatment is carried out directly on the solid material obtained from the pre-treatment phase. Under these conditions it is prolonged for considerably long residence times (about 30 days).
The stabilization is carried out when it is necessary to avoid the discharge of heavy metals possibly present, whereas the final dewatering has the purpose of reducing the volume of material treated.